1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices, and more particular, to a common voltage source integrated circuit (IC) for an LCD device that reduces a common voltage delay and prevents a block dim and a waving noise in common voltage swings.
2. Discussion of the Related Art
Until recently, display devices generally employed cathode-ray tubes (CRTs) or television monitors. Presently, many efforts are being made to study and develop various types of flat panel displays, such as liquid crystal display devices (LCDs), plasma display panel (PDPs), field emission displays, and electro-luminescence displays (ELDs), as substitutions for CRTs because of their high resolution images, lightness, thin profile, compact size, and low voltage power supply requirements.
In general, an LCD device includes a plurality of pixels arranged in a matrix, and each of the pixels has red-color, green-color, and blue-color sub-pixels. In addition, in a quad-type display device, each pixel has red-color, green-color, blue-color and white-color sub-pixels. During an operation of the LCD device, the gate lines are sequentially driven, thereby sequentially driving thin film transistors formed in the pixels row-by-row, while a data voltage is applied to the thin film transistors. At this time, the data voltage is inversed to a common voltage at every frame in order to change a direction of an electric field because if the electric field is continuously applied in the same direction the liquid crystals are deteriorated. Such a driving method of changing the polarity of data voltage is referred to as an inversion driving method.
FIG. 1 is a circuit diagram of an LCD device according to the related art. In FIG. 1, a liquid crystal display (LCD) device generally includes a liquid crystal panel 2 having a plurality of pixels P arranged in a matrix. The liquid crystal panel 2 includes a plurality of gate lines GL1 to GLm, where m is an integer, and a plurality of data lines DL1 to DLn, where n is an integer. The data lines DL1 to DLn are substantially perpendicular to the gate lines GL1 to GLm. Further, the LCD device includes a data driver 4 connected to the data lines DL1 to DLn and a gate driver 6 connected to the gate lines GL1 to GLm. The LCD device also may include a gamma voltage generator 8 connected to the data driver 4. A thin film transistor is disposed near each crossing of the gate and data lines. The gate driver 6 applies scanning signals to the gate lines GL1 to GLm to sequentially drive the thin film transistors row-by-row.
FIG. 2 is a circuit diagram of a common voltage source IC in the LCD device shown in FIG. 1. In FIG. 2, the common voltage source IC according to the related art includes an operational amplifier AMP and a push-pull circuit P/P. The operational amplifier includes an inverting input (−), an non-inverting input (+), and an output. A control signal CNT is applied to the inverting input (−) of the operational amplifier via a first resistor R1. In addition, the push-pull circuit P/P is connected to the operational amplifier output and a ground source. The push-pull circuit P/P receives a liquid crystal drive voltage VLCD and may output a common voltage Vcom.
Further, the common voltage source IC includes a variable resistor Rv and a driving resistor Ru. The variable resistor Rv is connected to the non-inverting input (+) of the operational amplifier AMP and the ground source. The driving resistor Ru is connected to the non-inverting input (+), and receives the liquid crystal drive voltage VLCD. The common voltage source IC device also includes a capacitor CF and a feedback resistor RF. The capacitor CF and the feedback resistor RF are parallel to each other and are connected between the output of the push-pull circuit P/P and the inverting input (−) of the operational amplifier AMP.
FIG. 3 is a waveform diagram of a control signal applied to the common voltage source IC shown in FIG. 2, and FIG. 4 is a waveform diagram of a common voltage output from the common voltage source IC shown in FIG. 2. As shown in FIG. 3, a control signal having a square waveform may be applied to the common voltage source IC (shown in FIG. 2). The common voltage can be calculated by the following equation (1) in accordance with the principle of inverting amplifier.
                    Vcom        =                                            -                              RF                R1                                      ×            CNT                    +                      Rv            ×                          VLCD                              (                                  Ru                  +                  Rv                                )                                                                        Equation        ⁢                                  ⁢                  (          1          )                    where VLCD is a liquid crystal drive voltage.
As shown in FIG. 4, when the common voltage Vcom is rising and falling, a signal delay (D) occurs due to a load represented by (RF//R1)×CF and a parasitic load parasitized on the lines of the liquid crystal panel. (RF//R1) is a resultant resistor value of the resistors RF and R1 that are connected in parallel.
In order to achieve the proper operation of the liquid crystal display device, the common voltage should reach its highest or lowest point within a blank time. That is, the common voltage swings, such as the common voltage rising and falling, should be performed within the blank time. However, as shown in FIG. 4, the common voltage source IC of the related art does not output the common voltage Vcom properly in time when the data voltage is applied. That is, the common voltage Vcom does not reach its lowest or highest point during the blank time, thereby casing a signal delay (D). As a result, the image quality of the liquid crystal display device is deteriorated. For example, the brightness of the LCD device is lowered, and the block dim and ripple noise are produced during the operation of the LCD device.